Various electro-optical systems have been developed for reading optical indicia, such as bar codes. A bar code is a coded pattern of graphical indicia comprised of a series of bars and spaces of varying widths, the bars and spaces having differing light reflecting characteristics. Some of the more popular bar code symbologies include: Uniform Product Code (UPC), typically used in retail stores sales; Code 39, primarily used in inventory tracking; and Postnet, which is used for encoding zip codes for U.S. mail. Systems that read and decode bar codes employing charged coupled device (CCD) or complementary metal oxide semiconductor (CMOS) based imaging systems are typically referred to hereinafter as imaging systems, imaging-based bar code readers, or imaging scanners.
Imaging systems electro-optically transform the graphic indicia into electrical signals, which are decoded into alphanumerical characters that are intended to be descriptive of the article or some characteristic thereof. The characters are then typically represented in digital form and utilized as an input to a data processing system for various end-user applications such as point-of-sale processing, inventory control and the like.
Imaging systems that include CCD, CMOS, or other imaging configurations comprise a plurality of photosensitive elements (photosensors) or pixels typically aligned in an array pattern that could include a number of arrays. The imaging-based bar code reader systems employ light emitting diodes (LEDs) or other light sources for illuminating a target object, e.g., a target bar code. Light reflected from the target bar code is focused through a lens of the imaging system onto the pixel array. As a result, the focusing lens generates an image from its field of view (FOV) that is projected onto the pixel array. Periodically, the pixels of the array are sequentially read out creating an analog signal representative of a captured image frame. The analog signal is amplified by a gain factor, by for example, an operational amplifier. The amplified analog signal is digitized by an analog-to-digital converter. Decoding circuitry of the imaging system processes the digitized signals representative of the captured image frame and attempts to decode the imaged bar code.
As mentioned above, imaging scanners typically employ an illumination system to flood a target object with illumination from a light source. Light from the light source is reflected from the target object. The reflected light is then focused through a lens of the imaging system onto the pixel array, the target object being within a field of view of the lens. It is not uncommon for a single imaging scanner to employ as an illumination source multiple LEDs or cluster of LEDs for producing illumination that is reflected from the target object. Each of the LEDs may need to be driven somewhat simultaneously with a current pulse in order to provide the illumination for the imaging scanner. In some embodiments, each LED may even be overdriven with a current pulse which has a peak current that is significantly higher than its rated DC operating current. When large number of LEDs are used, the total peak current required to drive all of the LEDs somewhat simultaneously with a current pulse may exceed the maximum current that can be supplied by a power source.
Accordingly, there is a need for a method and apparatus for providing current pulses to the light source in an imaging scanner in a manner that the current supplied by the power source can be used more effectively.